Try going for a run, or even a couple of trips upstairs, wearing a 30kg backpack and it soon becomes obvious how much hard work it takes to lug that extra weight around.
Lightweight technology is even more pressing with the rise of EVs, as the multiple-award-winning Jaguar I-Pace demonstrates. It makes no sense to build an EV with a heavier steel chassis and body and then compensate for that weight with a bigger, heavier battery.
As far as fuel consumption goes, a lighter car needs less energy to move it: it’s as simple as that. Pushing a classic Mini on your own is easy but moving a full-sized SUV solo is anything but. So manufacturers are always looking to reduce weight or at least reduce the weight spiral, mainly by using lightweight steel and aluminium alloy in the body construction. Although far cheaper than carbonfibre, aluminium construction is expensive, partly because of the material itself and partly because of the construction methods. That’s why it only appears to any great extent in premium cars.
Steel is a relatively easy material to work with and panels can be pressed and folded with crisp, sharp edges. Aluminium panels are less amenable to sharp folds, and if the radii on a swage line are too small or a shape too intricate, the material can crack. Designers and engineers have to work more closely to achieve the shapes designers want.
Joining the stuff together gets trickier as well. Whereas manufacturers have been joining steel for decades using MIG or spot welding, bolting and, to a lesser extent, riveting, some different methods are needed for aluminium. A favourite for joining panels is bonding (gluing) and riveting using self-piercing rivets. A conventional rivet is pushed through a drilled hole and bashed over, but self-piercing rivets push through the first layer of aluminium without drilling and splay out into a second layer but without piercing it. In cross-section, it resembles a wisdom tooth with those large roots and once in, like the tooth, it doesn’t want to come out.